Without limiting the scope of the invention, its background is described in connection with marine seismic exploration of the earth's strata located below a body of water, usually offshore, as an example.
Heretofore, in the field of marine seismic exploration, seismic prospecting of hydrocarbons and other natural resources found in relatively deep formations of the earth's crust has required the use of offshore seismic survey data. Suitable offshore structures, such as, survey vessels, drill rigs, barges, or permanent platforms for drilling and production operations have been employed not only for drilling and removal of hydrocarbons, but also as locations for obtaining engineering or exploration survey data about the location and composition of the layers that form the earth's strata therebelow.
Seismic prospecting has been used in the industry to obtain the geological data necessary for directing drilling operations. Both in principle and in theory, seismic prospecting is relatively straightforward. A pulse of seismic energy is produced from, for example, the abrupt discharges of explosives or air guns which are transmitted through the earth's strata. The reflected energy signals from the subsurface strata and strata interfaces are detected and recorded by suitable instrumentation. The data recorded by the instrumentation is recorded and/or processed, for example, using source signal deconvolution to obtain a profile of the characteristics of the subsurface strata. The data provides information relating to depth, arrangement and thickness of the various layers or formations of the strata as well as information about the composition of the layers.
Numerous efforts and approaches have been used to improve the desired characteristics of seismic pulses for source signals to generate seismic energy that is used to characterize earth's strata. The background art in this area has been extensively summarized in, for example, U.S. Pat. Nos. 4,956,822 and 5,469,404 to Barber, et al.,
Although much has been written about seismic pulse or source signal creation and interpretation, present methods for the detection and processing of reflected energy signal data to predict the details of strata or formations under investigation remains extremely complex and quite difficult. The calculations are complex and difficult because each seismic energy source produces an energy signal that has unique characteristics or signature. In deconvolution, the signature of the signal are used to adjust the recorded data for those known imperfections in the seismic signal. Separating true reflected seismic signals in the recorded data from noise or other signal echos is an extremely difficult task and requires a great deal of skill and expertise.
In addition to providing information for developing a drilling program, it has been found that seismic data can be used throughout the life of a hydrocarbon field. For example, seismic data may be used to determine the rate of depletion or the rate and level of water encroachment. It has been found, however, that typical seismic exploration which is conducted by towing a seismic source behind a marine vessel may not produce seismic data that can be correlated with prior seismic data. This lack of repeatability is due, in part, to the inability to generate the seismic signal from the same location as that in a prior survey. Also, the use of different equipment to produce the seismic signal requires different deconvolution routines which adds to the uncertainty of the resulting data.
A need has therefore arisen for a high resolution marine seismic signal source that may be used throughout the life of a hydrocarbon field to provide geological data relating to the earth's strata there below that may be correlated with prior survey data to determine, for example, formation depletion. A need has also arisen for such a system that may be positioned in a precise and repeatable location such that the location of the seismic signal is fixed over time.